1. Field of the Invention
The present invention relates to a distance measuring sensor detecting a position and a tilt of an object, a distance to the object and the like, by projecting a light on the object and receiving the light reflected therefrom, and a method for manufacturing such a distance measuring sensor.
2. Description of the Background Art
As a device for measuring a distance to an object, a distance measuring sensor applying a so-called triangle measuring method is known. FIG. 11 is a schematic view of the distance measuring sensor applying a so-called triangle measuring method, which should be referred in a following description of the principle of distance measurement thereof. A pulsed light output from a light emitting element, i.e., a light emitting diode (LED) 101, becomes a narrow beam through a projection lens 133, and projected on an object 150 or 160. The light reflected from object 150 or 160 is condensed on a light receiving face of a light receiving element, i.e., a semiconductor position sensitive detector (PSD), by a condenser lens 138.
Here, as shown in FIG. 11, a condensing position (spot position) where reflecting light is condensed by condenser lens 138 may vary in accordance with the distance from the distance measuring sensor to the object. By arranging the light receiving face of PSD 102 so as to cover the variation range of the condensing position, and processing a pair of photocurrent outputs from PSD 102, the distance to the object can be measured. It should be noted that a divided type photodiode having a plurality of light receiving faces may be used as a light receiving device other than the PSD.
FIG. 12 is a cross-sectional view showing a structure of a conventional distance measuring sensor using above-mentioned triangle measuring method. In conventional distant measuring sensor 110, LED 101, PSD 102, and a control IC 103 are mounted by die bonding, wire bonding or the like, on a lead frame 108. A translucent resin 109 is molded over these elements. Further, a case 111a consisting of an opaque resin is molded over translucent resin 109. Here, optical windows for passing lights are provided at least to the upper face of case 111a facing to LED 101 and PSD 102. A lens case 111b in which projection lens 133 and condenser lens 138 are integrally molded with the translucent resin is attached on case 111a. 
FIG. 13 is a circuit diagram of the conventional distance measuring sensor. A clock pulse having a prescribed period from an oscillator circuit arranged in a signal processing circuit 106 is provided to a timing generator circuit similarly arranged in signal processing circuit 106, and thus a drive pulse is generated. The drive pulse is input to light emitting circuit 104, and then LED 101 emits light.
A pair of feeble photocurrent outputs obtained by PSD 102 sensing the reflecting light is amplified by an amplifier circuit 105 and input to signal processing circuit 106. An operation processing based on this input signal is performed in signal processing circuit 106, and the result thereof is output to the outside via an output circuit 107. Normally, light emitting circuit 104, amplifier circuit 105, signal processing circuit 106, and output circuit 107 are integrally packaged in one control IC 103.
As for signal processing schemes, there are an analog output scheme in which an output value fluctuating in accordance with the distance, as shown in FIG. 14, is provided as information of a distance to an object, and an H/L output scheme in which an output value and a preset threshold value are compared and the result is output as a high (H) or a low (L) pulse.
On the other hand, in either output scheme, errors exist between the actual distance to an object and the output value of the distance measuring sensor. The errors may occur due to (1) variation in attaching position precision of LED, the projection lens, the condenser lens, and PSD, and (2) variation in element characteristics such as light emitting characteristics of LED, and light receiving characteristics of PSD.
For example, in the analog output scheme, such variations result in an output characteristics curve D or E shown by dotted lines in FIG. 14 that varies relative to a reference output characteristics curve C shown by solid line. Thus, errors occur between the actual distance to an object and the output value of the distance measuring sensor due to the above-mentioned variation in attaching position precision and variation in element characteristics. It applies to the H/L output scheme as well.
As a method for reducing the errors in output, one possible method is to measure the distance, after completing assembly of a distance measuring sensor, to an actually installed object with the distance measuring sensor and adjusting the obtained output value to a correct value.
For example, as shown in FIG. 13, one of the possible method is to arrange an external variable resistor 107a connected to output circuit 107 beforehand, and adjust the resistance value R of external variable resistor 107a to electrically adjust the output value for reducing the errors. On the other hand, the external circuit is required separately when using this method, which increases the manufacturing costs.
Another possible method is to correct the output of the distance measuring sensor with a microcomputer and the like by a user, without any adjustment in the manufacturing process of the distance measuring sensor. This method, however, will be a burden to a user, and therefore not preferable.